1. Technical Field
The present invention relates to a positioning system for use in GPS (Global Positioning Systems) assisted with mobile telephone networks for positioning.
2. Description of the Related Art
Global positioning systems, or GPS, have been available. In GPS, signals sent from satellites are received to determine the distance to the satellites to determine the position of the receiving point. In GPS, therefore, it is necessary to determine the position and time at which the satellites sent the signals. Satellite position information is sent from the satellites as almanac and ephemeris information, and can be acquired by picking up the signal from the satellites; however, it takes about 15 minutes to receive all of the information. Since it is difficult to supply accurate time information, generally, signals from four satellites are received to determine the position and time by solving simultaneous equations with four unknowns, that is, three-dimensional position coordinates and time. In the related art, there has been a network-assisted GPS in which information necessary for positioning, such as satellite position information and time information, is obtained not via satellite signals but via information in another way, such as a mobile telephone network, so as to greatly improve the positioning time and accuracy. Network-assisted GPS positioning of WCDMA mobile phones has been standardized by the 3GPP (Third Generation Partnership Project) (see 3GPP Specification TS25.305 V5.4.0).
In GPS, it is necessary to receive at least four satellite signals in order to detect the three coordinates of position, i.e., latitude, longitude, and altitude, and the time, or to receive at least three satellite signals, assuming that the altitude is ground level, in order to detect the two coordinates of position, i.e., latitude and longitude, and the time. In bad visibility conditions for satellites, such as indoors or between buildings in cities, a required number of satellite signals are not necessarily receivable. In order to overcome such a situation, a positioning technology in which radio signals from telephone-network base stations are used as satellite signals is known, and is standardized as OTDOA (Observed Time Difference Of Arrival) positioning technology, as shown in 3GPP Specification TS25.305 V5.4.0. FIG. 3 is a diagram showing the principle of a typical network-assisted GPS specified in 3GPP Specification TS25.305 V5.4.0. An SAS (Stand Alone Serving Mobile Location Center) 12 receives GPS satellite signals to obtain almanac and ephemeris data necessary for detecting the position. The position of the SAS 12 is known, and the SAS 12 can further generate data for correcting positioning errors from the difference between the known position and the positioning result from the GPS signals. In some cases, the SAS 12 also provides a portion of the SMLC (Serving Location Center) functionality, which should be performed by an RNC (Radio Network Controller) 13a. The RNC 13a controls a base station 15a to perform sequence control or radio resource management necessary for positioning. The RNC 13a specifies necessary assistance data from the SAS 12 based on positioning capabilities of the UE (User Equipment) to be measured, and sends it to a UE 16 via the base station 15a according to the positioning sequence. The timing necessary for UE positioning can be determined using a pilot signal or the like sent from the base station 15a. In WCDMA, base stations and GPS satellites are not generally synchronous, and an LMU (Location Measurement Unit) installed in each base station measures the deviation from the GPS time. There are two types of LMUs, that is, type A for connection with the base station by air, and type B for connection with the base station via a line.
Therefore, in such a typical network-assisted GPS using WCDMA mobile telephone networks, a device (LMU) for measuring a time difference between a signal sent from a radio-access-network base station and a signal sent from a GPS satellite, which have no synchronous relation therebetween, is essential in each base station.
Typically, in OTDOA technology, since signals from base stations are not synchronous, the timing difference between the base stations must be measured by some method. Thus, an LMU is also required for each base station in order to detect the timing difference of signals between the base stations.
It is an object of the present invention for the base station to provide a low-cost and high-reliability positioning system capable of precisely providing time to the positioning target UE and of OTDOA positioning in asynchronous WCDMA mobile telephone networks without installing an LMU in each base station.
It is another object of the present invention to provide a positioning system capable of short-time and high-accuracy positioning without an LMU.